Thermal dye diffusion or sublimation transfer systems have been developed to obtain prints from pictures that have been generated electronically, for example, from a color video camera or digital camera. Details of this process and apparatus for practicing it are contained in U.S. Pat. No. 4,621,271 to Brownstein. Thermal dye diffusion or sublimation transfer works by transmitting heat through the donor from the backside to the dye-donor layer. When the dyes in the dye-donor layer are heated sufficiently, they sublime or diffuse, transferring to the adjacent receiving layer of the receiver element.
All imaging dyes are unstable to light to a greater or lesser degree. Dyes are known to photolytically degrade via a number of paths which often involve dye triplet states, radicals and/or singlet oxygen. The light degradation is further known to occur to different degrees depending upon the color of the dyes, i.e.: yellow, magenta, cyan, or mixtures thereof. Multiple dye light stabilizing compounds may be required to significantly improve overall thermal transfer image stability. Each individual dye light stabilizing compound may provide a small improvement which adds to the overall stabilization by a chosen set of materials. Thus, for any given stabilizing compound, any demonstrated improvement in light stability is highly desirable.
Formulations of dye materials and stabilizing compounds can result in different light fastnesses. For example, even if enhanced light fastness of a magenta dye is achieved, that of a cyan dye may not be achieved, resulting in poor color balance of images. In a thermal transfer recording material having formed images, dyes forming yellow, magenta and cyan images exist in an identical layer, in which a dye exhibiting poorest light fastness is affected by the other dyes, resulting in a hue shift in color mixing in the neutral or gray images, leading to apparently deteriorated images.
The stabilizing compound may be incorporated into the dye layer of a donor element, it may be incorporated into a separate donor element, it may be incorporated into a separate portion of a donor element with repeating areas of dyes, or it may be incorporated into the dye receiving element. Incorporation of individual stabilizing compounds has been described in the prior patent literature most notably: U.S. Pat. Nos. 4,705,522, 4,855,281, 5,288.691, 5,342,728, 5,618,773, 5,620,941, 5,627,129, and recently, U.S. Patent Publication 2005/0233902. The materials described in these publications include phenols, epoxy compounds, alkoxy aryl compounds, dialkoxy aryl compounds, trialkoxy aryl compounds, alkyl or cycloalkyl substituted alkoxy aryl compounds, sulfonamido substituted aryl compounds, and hindered amine light stabilizing compounds, for example. The addition of extra stabilizing compounds to a dye donor layer of a dye diffusion thermal transfer donor material can have deleterious effects. For example, the addition of stabilizer to the cyan donor layer will decrease the amount of cyan dye which can be coated in the donor material thus decreasing the maximum density of the cyan image in the receiver resulting from cyan dye transfer. Further the addition of stabilizer to the cyan layer will decrease the amount of cyan dye which will transfer from the donor to the receiver since the stabilizing compounds absorbs a significant part of the thermal energy used to transfer the dye from the donor to the receiver, thus, decreasing the maximum density of the cyan image in the receiver resulting from cyan dye transfer.
Several types of cyan dyes for dye diffusion thermal transfer materials have been previously described in patents. For example in U.S. Pat. No. 4,695,287 a 2-carbamoyl-4-[N-(p-substituted aminoaryl)imino]-1,4-naphthoquinone is described, in U.S. Pat. No. 5,024,990 a mixture of cyan dyes is described, in U.S. Pat. No. 5,134,115 a cyan azamethine dye is described, in U.S. Pat. No. 4,990,484 a mixture of sublimable dyes of specific wavelength and molecular weight is described, in U.S. Pat. No. 5,252,530 a dye carrier layer containing a specific dye is described, and in RE 34,737 a sublimable dye and a binder on a base film is described.
However, none of the prior patent literature addresses the problems of addition of light stabilizing dye compounds in a dye diffusion thermal transfer element.